A hologram display apparatus forms a hologram pattern on a spatial light modulator, and by irradiating an illumination light beam on the spatial light modulator, forms an optical wavefront of an object to be displayed, thereby displaying a three-dimensional image within an observer's field of view. In general, letting the angle formed between the incident light and the exiting light with respect to the hologram be θ, a hologram image can be formed by a wavefront that includes a light ray group satisfying the following relationship:θ<arcsin(λ/2p)  (1)(where λ is the wavelength of the light source, and p is the light modulation device pitch of the hologram).
Here, the spatial light modulator is a device in which numerous minute light modulation devices are arranged two-dimensionally. The spatial light modulator modulates the phase, intensity, and the like of light that passes through or is reflected by the light modulation devices. Examples of spatial light modulators used for generating a hologram pattern include a spatial light intensity modulator that modulates the spatial intensity distribution of the optical wavefront of an illumination light beam and a spatial light phase modulator (also referred to below as a phase modulator) that modulates the spatial phase distribution of the optical wavefront of an illumination light beam.
From Equation (1), the display angle of view of an observable hologram image is 2θ, i.e. the range from −θ to θ. By reducing the light modulation device pitch p of the hologram, the display angle of view 2θ can be increased. With current techniques, however, it is difficult to reduce the light modulation device pitch of a light modulator beyond a certain amount. The display angle of view 2θ is thus restricted to a narrow range. Therefore, achieving a wide display angle of view is a practical challenge for hologram image display apparatuses.
On the other hand, it has been proposed to use a plurality of plane waves with different angles of incidence relative to the hologram in order to reproduce a hologram image with a display angle of view greater than 2θ relative to the hologram plane while satisfying relational expression (1) (for example, see NPL 1).